1. Field of the Invention
This invention relates to a Fischer-Tropsch process using a catalyst comprising a mixture of iron carbide and ilmenite supported on titania. More particularly, this invention relates to a process for producing substantially alkane hydrocarbons from mixtures of CO and H.sub.2 using a catalyst comprising a mixture of iron carbide and ilmenite supported on titania wherein the ratio of the iron present in said supported iron carbide and ilmenite, calculated as Fe.sub.2 O.sub.3, to the surface of the titania support ranges from about 2 to 25 milligrams per square meter of titania support surface area.
The use of iron-titania mixtures as Fischer-Tropsch catalysts for converting mixtures of CO and H.sub.2 to hydrocarbons is well-known to those skilled in the art. For example, U.S. Pat. No. 2,543,327 discloses titania promoted iron oxide for Fischer-Tropsch synthesis wherein the iron oxide is in the form of naturally occurring magnetite and preferably as Alan Wood ore. In this disclosure a typical catalyst is shown as prepared by mixing about 13,600 grams of Alan Wood ore with 98 grams of titania and 216 grams of potassium carbonate used as a promoter. The ratio of hydrogen to carbon monoxide disclosed as being preferably at least 2/1 and the results show that the catalyst has relatively poor activity with a large selectivity towards the production of methane and very little selectivity towards the production of C.sub.2.sup.+ hydrocarbons. That is, the Fischer-Tropsch product was primarily methane. Similarly, British patent No. 1,512,743 also discloses a titania promoted, massive iron type of Fischer-Tropsch catalyst wherein iron oxide is mixed with titanium oxide, zinc oxide and potassium carbonate with the resulting mixture being sintered and then reduced for many hours at 500.degree. C. Although this catalyst has relatively reasonable activity with regard to conversion of the CO and H.sub.2 mixture, the product was primarily (i.e., about 73%) olefinic, unsaturated C.sub.2 /C.sub.4 hydrocarbons and with only about 10% of C.sub.2 /C.sub.4 saturated hydrocarbons or alkanes being produced. U.S. Pat. Nos. 4,192,777 and 4,154,751 while directed towards the use of potassium promoted Group VIII metal cluster catalysts in Fischer-Tropsch synthesis reactions, suggest that iron supported on titania would be useful Fischer-Tropsch catalysts but do not disclose the preparation of same. In their examples, they disclose iron on various supports other than titania with the amount of iron on the support generally being less than about 5 percent. U.S. Pat. No. 4,261,865 discloses an iron titanate-alkali metal hydroxide catalyst for preparing alpha-olefins from mixtures of CO and H.sub.2. That is, the catalyst is not iron supported on titania along with an alkali metal hydroxide but rather an iron titanate compound.
Another example of a titania-promoted massive iron catalyst for Fischer-Tropsch synthesis may be found in the Volume 17, No. 3-4 React. Kinet. Catal. Lett., pages 373-378, (1981) titled "Hydrocondensation of CO.sub.2 (CO) Over Supported Iron Catalysts". This article discloses an iron oxide, titania, alumina, copper oxide catalyst promoted with potassium. Similarly, in European patent application No. EP 0 071770 A2 Fischer-Tropsch catalysts are disclosed which include iron titania catalysts wherein the iron to titania ratio can be greater than 1/10. The actual iron-titania catalyst is not an iron supported on titania catalyst but an iron/titania catalyst produced by a coprecipitation technique wherein the active iron catalytic component is distributed throughout a titanium oxide matrix. Thus, the resulting catalyst was not iron supported on titania but rather a bulk phase iron/titania mixture which, when used for Fischer-Tropsch synthesis, produced predominantly olefins. The amount of olefins produced was generally greater than about 80% of the total hydrocarbon product.
With regard to iron/titania catalysts for Fischer-Tropsch wherein the iron is supported on titania, a 1982 article by Vannice, Titania-Supported Metals as CO Hydrogenation Catalysts, J. Catalysis, v. 74 p. 199-202 (1982) discloses the use of an iron/titania catalyst for Fischer-Tropsch synthesis wherein the amount of iron, calculated as metallic iron, is 5 percent of the iron/titania composite and the catalyst shows extremely little activity for Fischer-Tropsch synthesis.
An article by Reymond et al, Influence of the Support or of an Additive on the Catalytic Activity in The Hydrocondensation of Carbon Monoxide by Iron Catalysts in "Metal-Support and Metal-Additive Effects in Catalysis, B. Imelik et al. (Eds), Elsevier, Netherlands, p. 337-348 (1982) also discloses the use iron/titania Fischer-Tropsch catalysts wherein the iron is supported on the titania. The iron/titania catalysts disclosed contain about 9.5 weight percent iron on titania and the activity of the resulting catalysts is presented as a function of the activation pretreatment of the iron/titania catalyst precursor. Thus, it was disclosed that if the precursor was pretreated in either helium or hydrogen at 250.degree. C. there was relatively little activity for Fischer-Tropsch synthesis. Similarly, another composite treated in hydrogen for 15 hours at 500.degree. C. showed no activity whatsoever. It is important to note that the catalytic activity was expressed only as a function of methane production using a 9/1 mole ratio of H.sub.2 /CO at one atmosphere pressure and a reaction temperature of 250.degree. C.